Method for efficient and environmentally clean utilization of solid fuels

ABSTRACT

A process of oxidizing an ash and sulfur containing fuel such as coal in order to power gas turbines using a material such as the oxides of iron which in an oxidized state can be readily reduced and which in a reduced state is readily oxidized. Preferably, the oxides of iron are circulated between two fluid bed reactors and reduced by the ash and sulfur containing fuel in the first said fluid bed reactor and oxidized by air in the second said fluid bed reactor, with ash and SO 2  in the gases leaving chiefly from the first said fluid bed reactor. The temperature is controlled within the second said fluid bed reactor by use of a clean fuel and the rate of addition of the ash and sulfur containing fuel to the second fluid bed is controlled so as to limit the amount of SO 2  in the gases leaving the second said fluid bed.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an improved method for using biomassand fossil fuels, such as coal, in order to power gas turbine enginesusing unmixed combustion of solid fuels. The invention also relates to aprocess for separating the products of unmixed combustion, includingpollutants such as carbon dioxide, sulfur compounds, nitrogen compounds,and volatile metals (e.g., mercury) into a separate stream available forsubsequent treatment and ultimate sequestration.

[0002] One of the major problems in modern industrial society is theproduction of air pollution by conventional combustion systems based onbiomass and fossil fuels. The oldest recognized air pollution problem isthe emission of smoke. In modern boilers and furnaces, smoke emissionscould be eliminated or at least greatly reduced by the use of Over FireAir (“OFA”) technology. Other types of air pollution produced bycombustion include particulate emissions such as fine particles of ashfrom pulverized coal firing, oxides of sulfur (SO₂ and SO₃), carbonmonoxide emissions, volatile hydrocarbon emissions and the release oftwo oxides of nitrogen, NO and NO₂. More recently, the problem of globalwarming due to greenhouse gas emissions of CO₂ from power plants andother combustion systems have become a matter of serious environmentalconcern.

[0003] Another major technological problem concerns the use of coal as afuel for powering gas turbines. Gas turbines are the lowest capital costsystems available for generating electrical power. Since thethermodynamic efficiency of gas turbines increases with increasingturbine inlet temperature, efforts to improve turbine efficiencygenerally involve increasing the turbine inlet temperature to higherlevels. As a result, turbine blades and other components have beenengineered to tolerate increasing high inlet temperatures.

[0004] It is well known that the hot gases produced by coal firingcontain fly ash (which is erosive to turbine blades). In the presence ofthis erosive fly ash the maximum service temperature at which turbineblades can operate is less than it would be otherwise. This limitationsignificantly decreases the overall process efficiency and lowers thecompetitiveness of coal as a gas turbine fuel. These and otherdisadvantages have also prevented lower cost (and abundant) coal frombeing considered an attractive gas turbine fuel. If a process weredeveloped whereby coal could be burned in a manner that produced hotgases that were not erosive or corrosive, the need for temperaturereduction would be eliminated and coal would become a much moreeconomically viable gas turbine fuel.

[0005] With respect to global warming, coal has the further disadvantagethat its CO₂ emissions per BTU released are significantly higher thanthose of most ashfree fuels. Again, however, if coal could be burned ina manner that did not cause the emission of CO₂ and/or other pollutants,this known disadvantage would disappear, making coal a much moreenvironmentally acceptable fuel for existing uses and new uses such asfueling gas turbines.

[0006] U.S. Pat. Nos. 5,339,754, 5,509,362 and 5,827,496 (incorporatedherein by reference) disclose a new method of burning fuels using acatalyst that is readily reduced when in an oxidized state and readilyoxidized when in a reduced state, with the fuel and air beingalternatively contacted with the catalyst. The fuel reduces the catalystand is oxidized to CO₂ and water vapor. In turn, the air oxidizes thecatalyst and becomes depleted of oxygen. Combustion can thereby beeffected without the need of mixing the fuel and air prior to or duringthe combustion process. If means are provided whereby the CO₂ and watervapor and the oxygen depleted air can be directed in differentdirections as they leave the combustion process, the mixing of fuel andair can be completely avoided. This particular method of combustion hasbecome known in the art as “unmixed combustion.” In one embodimentdisclosed in the '362 patent, the CO₂ produced by the combustion processis separated from the water vapor and disposed of by conventional means.The '362 patent also removes the acid gases such as SO₂, HCl and HF.

[0007] It is well known that the total volume of combustion gasesproduced by unmixed combustion is comparable to that produced inconventional combustion. It is also well known that the cost of removingacid gases from combustion effluents by scrubbing increases with thevolume of gas being scrubbed. The '362 patent recognize that if unmixedcombustion is carefully controlled such that the acid gases leave thecombustion process as part of the CO₂ and water vapor steam, the volumeof gas that must be scrubbed can be greatly reduced, as well as the costof scrubbing.

[0008] The subject matter of the '362 patent is discussed in greaterdetail in paper 98F36, presented at the October 1998 meeting of theWestern States Section of the Combustion Institute (hereafter referredto as the “Combustion Institute paper”). The authors of the paperinclude R. K. Lyon (the inventor of U.S. Pat. No. 5,509,362 and theinventor of the present invention) and J. A. Cole. The paper discloses aconceptual process for using coal to power a gas turbine and reports ona series of experiments illustrating certain aspects of the proposedprocess.

[0009] The reported experiments used an atmospheric pressure fluid bedof powdered, chemically pure iron oxide (i.e., FeO/Fe₂O₃). In theexperimental setup, the gas being used to fluidize the bed could beswitched from air to 5% SO₂+95% N₂ and back again. The basic experimentsas reported in the paper involved two steps. First, a bed fully oxidizedto Fe₂O₃ was fluidized with the 5% SO₂+95% N₂ at a temperature of 857 @C. A small amount of coal was injected into the bed while the gasesleaving the bed were continuously analyzed. In a second step, thefluidizing gas was switched to air while the gases leaving the bed wereanalyzed. Based on available data, the paper concludes that coal isreadily oxidized in the presence of SO₂ and that the chief carboncontaining product of the oxidation is CO₂, with little or no CO beingproduced. The paper attributes the ability of the solid particles ofFe₂O₃ to rapidly oxidize the coal to a catalytic action by the SO₂ usedin the fluidizing gas. That is, the SO₂ reacts with the coal, convertingit into to CO₂, CO, CS₂, COS, and sulfur vapor. The CO, CS₂, COS, andsulfur vapor are, in turn, oxidized by the Fe₂O₃ to CO₂ and SO₂. Thus,the SO₂ serves as a catalyst, allowing the solid Fe₂O₃ to oxidize thesolid coal char. The first half of this process, the gasification ofcoal char by SO₂, is described by J. D. Blackwood and D. J. McCarthy inthe Australian J. Chem. P. 723, 1973.

[0010] The initial experiments reported in the Combustion Institutepaper indicate that the gases exiting the bed after being fluidized withair contain little or no SO₂ and little or no CO and CO₂. Thus, thepaper concludes that the Fe₂O₃ oxidized the coal to completion duringthe first step, i.e., while the bed was fluidized with 5% SO₂+95% N₂.The oxidation converted all the sulfur in the coal to SO₂ and othervolatile species which exited the bed during the first step of theexperiment.

[0011] Another series of two-step experiments discussed in the paperused a bed fluidized with N₂. Like the experiments conducted with coal,when the amount of thiophene injected was small, all of the sulfur leftthe bed as SO₂ and other volatile species during the first step.Conversely, none of the sulfur was retained in the bed during the firststep and exited during the second air fluidization step. Increasing theamount of injected thiophene changed that situation. That is, injectingthiophene in excess of a threshold amount caused some of the sulfur tobe retained in the bed during the first step and to be released as SO₂during the second air fluidization step. The paper speculates that thisthreshold is a result of FeS, i.e., after thiophene reduces some of theFe₂O₃ to FeO, injection of more thiophene causes the formation of FeS.Once formed, the FeS remains during the first step and then oxidizes toFe₂O₃ and SO₂ during the air fluidization step.

[0012] Based on these experimental observations, the CombustionInstitute paper then proposes a conceptual design for a process to usecoal to power a gas turbine. As shown in The reference's FIG. 4, theFe₂O₃ catalyst in fluidized powder form circulates between a first fluidbed fluidized with steam and a second bed fluidized with air. FIG. 4shows the transfer lines between two fluid beds as being purged withsteam. The second fluid bed is fluidized with compressed air from thecompressor section of a gas turbine. With this bed, FeO is oxidized toFe₂O₃, a strongly exothermic reaction that depletes the compressed airof oxygen and heats it. The heated compressed air is then used to drivethe expander section of the gas turbine.

[0013] The Combustion Institute paper contemplates feeding pulverizedcoal to the first steam fluidized bed where it reduces the Fe₂O₃ to FeOwhile being oxidized to CO₂, water vapor, and fly ash. All the volatileproducts of combustion are swept from the bed. The fluidizationconditions in the bed are such that the fly ash is rapidly removed fromthe bed by elutriation. FIG. 4 calls for the fly ash to be removed fromthe other combustion products with a cyclone separator after which theash goes to disposal. Once heat is recovered from the remainingcombustion products, water vapor is removed by condensation and theresultant CO₂ and SO₂ mixture is disposed of.

[0014] The Combustion Institute paper concludes that the conditionsunder which the coal is oxidized are such that all or virtually all ofthe sulfur in the coal is converted to SO₂ and other volatile speciesrather than reacting with the FeO/Fe₂O₃ to form FeS or other nonvolatilesulfur containing species. Obviously, the formation of FeS and similarspecies is undesirable from an environmental standpoint. Instead ofbeing swept out of the steam fluidized bed, they tend to circulate intothe air fluidized bed where they oxidize to SO₂ and cause the emissionof air pollutants.

[0015] Three final aspects of the Combustion Institute paper should alsobe noted. First, the paper teaches that if the conversion of Fe₂O₃ toFeO is kept below a certain threshold, FeS is not formed and SO₂emissions are avoided. Although the paper notes the amount of thiopheneneeded to exceed this threshold for the amount of Fe₂O₃ used in theexperiment, it is silent as to how much Fe₂O₃ was used. Thus the paperdoes not identify the extent of Fe₂O₃ conversion at which the thresholdoccurs. Nor does it explain how a change in temperature effects thethreshold or how catalyst aging changes the threshold. The paper alsofails to disclose how the threshold would be effected by changing theform in which the Fe₂O₃ is used, e.g., replacing chemically pure Fe₂O₃with iron ore, red mud (a byproduct of aluminum production with a highiron content) or other low cost iron containing products.

[0016] With respect to the problem of achieving complete combustion ofthe coal, the Combustion Institute paper teaches that 5% SO₂ as used inthe paper's experiment corresponds to the concentration of SO₂ producedin the proposed process, i.e., the concentration of SO₂ in moles perliter which would be produced by oxidation of high sulfur Illinois coalwith Fe₂O₃ at a sufficiently elevated pressure. For lower sulfur coals,the concentration of SO₂ will be lower, making the coal oxidation rateunacceptably low. The only solution suggested for this problem is veryexpensive, namely raising the SO₂ concentration by recycling SO₂, i.e.,by recovering SO₂ from the recovered SO₂+CO₂ mixture and returning it tothe first fluid bed.

[0017] It is well known that efficient coal combustion requires that thecarbon content of the fly ash be low. The Combustion Institute paper'sexperiments show that coal can be rapidly oxidized to CO₂, water vaporand “fly ash.” However, a coal particle becomes “fly ash” when oxidationshrinks it to the point that it flies out of the fluid bed. While thisimplies that the “fly ash” would have a substantial carbon content, thepaper does not identify the carbon content of the fly ash. Thereference's FIG. 4 contemplates removing fly ash from the gases leavingthe first fluid bed with a cyclone and sending this fly ash to disposal.However, this would mean discarding a significant fraction of the coal'sheat of combustion.

[0018] It is also known that the theoretical maximum possible efficiencyof a gas turbine increases with increasing turbine inlet temperature.Thus, if a gas turbine is to operate with an acceptably high efficiency,the inlet temperature should be at temperatures approaching 1500° C. Forthe conceptual process shown in The reference's FIG. 4, the turbineinlet temperature would be the same or slightly less than thetemperature at which the second fluid bed operates. On page 10, thepaper teaches that the first fluid bed is to be operated at atemperature of 700° C.-900° C. Within the framework of the reference'sFIG. 4 conceptual process, this teaching is necessary since the firstfluid bed must be operated at a temperature below the coal's ash fusiontemperature.

[0019] The Combustion Institute paper teaches that the second fluid bedshould be operated at a temperature of “nearly 1500° C.” This teachingis necessary if the gas turbine is to operate with satisfactoryefficiency and implies a temperature increase of 600° C. to 800° C. Inorder to provide this temperature increase, the paper teaches that theratio of coal to Fe₂O₃ feed to the first bed be sufficient so that 60%of the Fe₂O₃ is reduced to FeO. One can readily calculate that if astoichiometric quantity of air is preheated to 400° C. and reactsadiabatically with Fe₂O₃ 60% of which has been reduced to FeO, the finaltemperature will be 1495.2° C.

[0020] However, important limitations exist with respect to the catalystunder such conditions. The threshold for FeS formation must be 60%conversion or greater if SO₂ emissions are to be avoided. The catalystmust consist almost entirely of Fe₂O₃/FeO, i.e., if the catalystcontained any substantial amount of inert material, the added heatcapacity of this inert material would reduce the temperature increase.The Combustion Institute paper thus requires the use of pure or nearlypure Fe₂ O₃/FeO, a relatively expensive material, rather than much lessexpensive iron ore or red mud. Furthermore, aging of the expensiveFe₂O₃/FeO catalyst effectively converts it into an inert heat capacity.Thus, the teachings of the paper imply that the catalyst life will beshort since relatively little catalyst aging can be tolerated. The paperalso confirms the disadvantage in having sulfur in the coal recovered asSO₂. There is virtually no market for sulfur as SO₂ and its storage anddisposal can be expensive and difficult. In contrast, elemental sulfuris readily shipped and has a substantial market potential. Moreover, insituations in which it cannot be sold, the storage and/or disposal ofsulfur is relatively easy and inexpensive.

[0021] Thus, despite recent developments in the art, a significant needstill exists in the art for a new method of burning coal to power gasturbines that will avoid the limitations discussed above with respect tothe unmixed combustion of solid fuels such as coal.

BRIEF SUMMARY OF THE INVENTION

[0022] The present invention provides an improved method of burning coalto power gas turbines, and achieves high efficiency while controllingSO₂, CO₂, Hg and NO_(x) emissions. The invention also provides animproved method of burning coal as compared to the prior art (includingthe Combustion Institute paper) that allows the use of catalysts otherthan high purity Fe₂O₃/FeO (i.e., iron oxides with a significantfraction becoming inert due to aging and in mixture with inertnoncatalytic materials). The invention also provides a method ofefficiently separating all the pollutants, including CO₂, sulfurcompounds, nitrogen compounds, and volatile metals, such as mercury,into a separate stream for downstream treatment or disposal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic diagram of a circulating fluidized bedsystem in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] With reference to FIG. 1, a mixture of coal and steam is fed to afirst fluidized bed reactor 10 and air is fed to a second fluidized bedreactor 12 (labeled “Regenerator” in FIG. 1). Coal or biomass issupplied to the first reactor 10 via inlet 14 (which can be an airlocktype inlet) at about the midpoint of fluidized bed 10, while steam issupplied to the bottom of the reactor via inlet 16. Air is supplied tothe bottom of the second reactor 12 via inlet 18. A catalyst containingiron oxides (FeO and Fe₂O₃) is circulated between the two fluidized beds10, 12 via crossover conduits 20, 22 in order to transfer oxygen presentin the system. The connections between the fluidized beds are isolatedby flowing steam via inlets 24, 26 to prevent the crossover of gases.

[0025] Steam for the first fluidized bed reactor 10 is primarilysupplied by a high pressure steam turbine 28 via stream 30. Some thesteam is supplied to a low pressure steam turbine 32 and then tocondenser 34. A portion of the steam is also extracted for supply to thecrossover conduits 20, 22 via respective streams 36, 38. In the firstfluidized bed 10, the Fe₂O₃ is reduced to FeO.

[0026] 27. As the coal particles react, they become smaller and arecarried to the top of the fluidized bed 10. Meanwhile, fresh Fe₂O₃ isintroduced at the top of the fluidized bed 10 and circulates throughoutthe bed as it is converted to FeO. At the bottom of the first fluidizedbed, the FeO is conveyed through a vapor lock and conduit 22 to the topof the second fluidized bed 12. The gas stream leaving the firstfluidized bed 10 is then fed to a cyclone separator (not shown) or otherconventional hot gas cleanup systems known in the art to remove ash. Ifthe gas stream is to be discharged to the atmosphere it may undergofurther purification or, if it is to be sent to sequestration it mayneed no further processing.

[0027] If the amount of MO is sufficient to complete the combustionprocess in the first fluidized bed 10, final combustion products (CO₂and H₂O) are formed. The product gas can be cooled in a heat recoverysteam generator (“HRSG”) 40 to produce steam. The product gas consistsof CO₂, H₂O, and SO₂ at elevated pressure, with small amounts of otherpollutants. The SO₂ and other pollutants can be removed by conventionalwet scrubbing or other treatment (also not shown), leaving anessentially pure stream 58 of pressurized CO₂ for sequestration ordischarge.

[0028] Alternatively, the SO₂ and other air pollutants can be removed ina gas cleanup system and the product gas expanded across gas turbine 42(labeled “Gas Turbine (optional)”) to produce electricity. The gas canthen be cooled in HRSG 40, and the product gas 58 (mostly CO₂), nowalmost at atmospheric pressure, discharged or sequestered.

[0029] As noted above, the second fluidized bed 12 in FIG. 1 isfluidized with air which oxidizes the FeO to Fe₂O₂. Clean fuel such asnatural gas can, depending on operating conditions, be added to the airvia line 70. The oxidation of this clean fuel in fluidized bed 12provides additional heat. The gas stream 50 leaving the second fluidizedbed 12 typically passes through a cyclone or other hot gas cleanupsystem (not shown) to remove elutriated metal oxides. The product gasfrom the second fluidized bed consists of vitiated air, which is thenexpanded across gas turbine 52. The second fluidized bed may consist ofa bubbling fluidized bed or a riser reactor.

[0030] The remaining enthalpy in the different streams outlined in FIG.1 can be recovered in HRSG units, thereby providing steam forfluidization, steam for purging the vapor locks and steam for generationof electricity in the steam turbines.

[0031] One of the potential advantages of the preferred method for coalconversion according to the invention is the ability to greatly limitthe formation of NO_(x). The oxidation of the clean fuel and of FeO toFe₂O₃ occurs at a temperature too low for formation of thermal NO_(x) bynitrogen fixation. If the clean fuel does not contain chemically boundnitrogen, there will be no formation of fuel NO_(x). In addition to thethermal NO_(x) and fuel NO_(x) mechanisms for forming NO_(x) there is aminor mechanism known as the prompt NO_(x) mechanism. NO_(x) formationby this mechanism is always small and since the clean fuel is used inlimited amounts, the formation of NO_(x) by this means is furtherlimited.

[0032] The present invention also contemplates means for efficientlycontrolling the systems for coal conversion, namely (1) continuousmonitoring the SO₂ content of the oxygen depleted air as it exits theturbine; (2) adjusting the rate at which coal is fed to the first fluidbed; (3) continuous monitoring of the temperature of the second fluidbed; and (4) adding a controlled amount of natural gas or other cleanfuel to the second fluid bed. In this context, the term “clean fuel”relates to the applicable federal emission regulations. That is, a fuelwith a relatively low sulfur content might not be considered a “cleanfuel” in regions in which emission regulations are extremely strict,even though it might be so defined in other regions.

[0033] A typical example of the continuous monitoring of SO₂ may bedescribed as follows. A signal from the SO₂ monitor is sent to themechanism controlling the coal input rate, causing it to increase thecoal input rate when the SO₂ level in the gases leaving the turbine isbelow an acceptable value, and to decrease the coal rate when the SO₂level exceeds the acceptable value. In this manner, the rate at whichcoal is fed to the first fluid bed (item 64 in FIG. 2) is adjusted tothe threshold of incipient FeS formation and maintained at thatthreshold despite variations in the value due to catalyst aging or othercauses.

[0034] 42. In like manner, a signal from the temperature monitor on thesecond fluid bed 66 is sent to the mechanism controlling the clean fuelinput rate, causing it to increase the clean fuel input rate when thebed temperature falls below a preset value, and to decrease the fuelrate when the bed temperature exceeds the preset value. The bed operatesunder conditions whereby the clean fuel readily oxidizes, deliveringheat to the said second bed in addition to that which is provided by theoxidation of FeO to Fe₂O₂. In this manner, the total rate of heatrelease in the second bed is held constant despite variations in theamount of heat produced by oxidation of FeO to Fe₂O₃ in the second fluidbed, thus maintaining the temperature of the gases going into the gasturbine at a constant high valve and ensuring better turbine efficiency.

EXAMPLE 1

[0035] Various experiments were conducted to reduce a sample of pureFe₂O₃ with thiophene at 732° C. and reoxidizing it with air. Forreductions of the Fe₂O₃ of less than 60%, the subsequent reoxidation didnot produce SO₂.

EXAMPLE 2

[0036] Heat balance calculations were done for the 60% reduction ofFe₂O₃ to FeO at 900° C. and for the subsequent reoxidation underadiabatic conditions with 150% stoichiometric air, the air beingpreheated to a temperature of 400° C. The oxygen depleted air was foundto exit the second fluid bed at 1487.5° C.

EXAMPLE 3

[0037] The heat balance calculation in comparative example 2 wasrepeated but with the pure Fe₂O₃ being replaced with a mixture of 50mole % Fe₂O₃, 50 mole SiO₂. The oxygen depleted air exited the secondfluid bed at 1337.70° C.

[0038] Comparative examples 2 and 3 show that the process described bythe Combustion Institute paper can generate gases hot enough toefficiently power a gas turbine, i.e., temperatures approaching 1500°C., but that this ability is lost (or substantially reduced) when thecatalyst is diluted with inert materials.

EXAMPLE 4

[0039] The heat balance calculation in comparative example 3 concernedthe case in which 3% CH₄ (natural gas) is added to the air going intothe second fluid bed. The oxygen depleted air was found to exit thesecond fluid bed at 1516.5° C. This illustrates that the addition of aclean fuel will maintain the temperature at a level consistent with highturbine efficiency while allowing the effective use of coal as the solidfuel.

[0040] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A process for using a fuel which contains ash andsulfur using an unmixed combustion catalyst capable of being reducedoxidized state and being oxidized when in a reduced state, comprisingthe steps of: circulating the unmixed combustion catalyst materialbetween two fluid bed reactors to cause the catalyst to become reducedby said ash containing fuel in the first fluid bed reactor and oxidizedby air in the second fluid bed reactor; removing ash and SO₂ gases fromsaid first fluid bed reactor; measuring the amount of SO₂ in the gasesleaving said second fluid bed reactor; controlling the rate of said ashcontaining fuel addition to said first fluid bed reactor based on themeasured amount of SO₂ in the gases leaving said second fluid bedreactor; and controlling the temperature within said second fluid bedreactor using a clean fuel.
 2. The process of claim 1 wherein said cleanfuel does not contain chemically bound nitrogen.
 3. The process of claim1 wherein said clean fuel is natural gas.
 4. The process of claim 1wherein natural gas is added to said air prior to the air entering thesaid second fluid bed.
 5. The process of claim 1 wherein the preferredoxygen transfer catalyst material is iron oxide.
 6. The process of claim5 wherein said metal oxides are used in the form of ore, ore wasteproducts, or ore byproducts and have a metal purity of less than 90%. 7.The process of claim 1 wherein the amount of the unmixed combustioncatalyst and residence time in the reactors are sufficient to providecomplete oxidation of said solid fuel.
 8. An apparatus for oxidizingcoal using an unmixed combustion catalyst that is capable of beingreadily reduced when in an oxidized state and readily oxidized when in areduced state, comprising first and second fluid bed reactors; means forcirculating a mixture of said coal and said oxygen transfer catalystmaterial between said first and second fluid bed reactors; means forremoving ash and SO₂ gases from said first fluid bed reactor; means formeasuring the amount of SO₂ in the gases leaving said second fluid bedreactor; a controller capable of adjusting the rate of solid fueladdition to said first fluid bed reactor based on the measured amount ofSO₂ in the gases leaving said second fluid bed reactor; and a controllerfor adjusting the temperature within said second fluid bed reactor usinga clean fuel.